I just bought a very slightly used 1996 Miracle 20 and the rudders hum louder than I can talk! The old plastic rudders on my H-16 did that and I fixed it by sharpening the rudders with a machete (it is easier to control than a pair of scissors). On my new boat the previous owner had trimmed the trailing edge of both the rudders and the daggerboards to a very rounded profile. I assume that this causes the hum. He trimmed one of the daggerboards so much that the core was exposed and it had started to delaminate. I fixed it with epoxy and so far the fix has held.

Question: Are my rudders also constructed with a core like the daggerboards? Do I need to be careful when reshaping the trailing edges so that I don't expose the core, or are they a solid material like the old H-16 plastic ones?

Foam core, so build up before you file and sand the trailing edge to a nice smooth exit point. To get rid of the hum, no hollows (bumps on the trailing edge), no round edges. A squared off edge is easier to maintain than a sharp point, either will eliminate the singing foils.

Now that I've reshaped the rudders they're quiet enough for me to hear the daggerboards humming. Same problem. Previous owner rounded the trailing edges. With the foam core I'm concerned about shaving them down too much to get the correct shape (he REALLY rounded them off). What do I use to build them up before reshaping them?

When the boards start singing to us we just take a sanding block and wet sandpaper out on the water with us, sail a bit, pull a board up and just a bit and then see if it helped, we do this until it is gone.

The flat trailing edge is exactly right. If it is rounded, the fluid flowing over the rudders and daggers will not separate cleanly and will cause cavitation (the hum). If you want futher explaination let me know. Otherwise I won't bore everyone here with a fluid dynamics lecture.

Rudder edge shaping tool? Nope. That really hacked the trailing edge anyway. You should use a file or sand paper on a block.

Au contraire mon ami. The shaping tool creates a smooth, consistent trailing edge. A number of friends tried scissors, knives, files, sand paper on a block, machete and guillotine with no improvement. They used the shaping tool and voila, no more hum.